The core of Warhammer 40000 is the game mission. This is what gives you the details of what you need to do to win a specific specific game. You can create missions yourself, and Games Workshop will publish missions in future army Codex and rule extensions, however the missions that you are likely to be playing most often are the ones published in the rule book. If you get familiar with the rule book missions then you will win more games!
In the 8th Edition rule book there are 5 different sections of missions starting at page 186. One thing you can not complain about is the lack of missions in this release! Each section of missions do get more complicated as you progress through the rules. If you are new to Warhammer 40k I recommend that you start with the ‘Only War’ missions on page 187 or the ‘War Without End’ missions on page 191.
Only War Missions
On page 187 you can find the Only War missions. At first look this page looks like there is just one mission, but on closer inspection there is a table where you roll to select one of three missions.
Each of the missions on this page use objective markers. You can use anything as an objective markers. Dice are popular, or coins work well. If you play WH40k regularly then creating terrain pieces numbered one to six looks better on the table.
The missions use objective markers, but do not mention how many to set up. I suggest that for small games up to 1000 points 4 markers work well, standard games of 1000 to 3000 points, 6 markets are recommended, and larger games need 8 markers, and the table space to place them!